The Government was accused of “breathtaking laxity” in its arms controls last night after it emerged that officials authorised the export to Syria of two chemicals capable of being used to make a nerve agent such as sarin a year ago.

The Business Secretary, Vince Cable, will today be asked by MPs to explain why a British company was granted export licences for the dual-use substances for six months in 2012 while Syria’s civil war was raging and concern was rife that the regime could use chemical weapons on its own people. The disclosure of the licences for potassium fluoride and sodium fluoride, which can both be used as precursor chemicals in the manufacture of nerve gas, came as the US Secretary of State John Kerry said the United States had evidence that sarin gas was used in last month’s atrocity in Damascus.

Mr Kerry announced that traces of the nerve agent, found in hair and blood samples taken from victims of the attack in the Syrian capital which claimed more than 1,400 lives, were part of a case being built by the Obama administration for military intervention as it launched a full-scale political offensive on Sunday to persuade a sceptical Congress to approve a military strike against Syria.

The Department for Business, Innovation and Skills insisted that although the licences were granted to an unnamed UK chemical company in January 2012, the substances were not sent to Syria before the permits were eventually revoked last July in response to tightened European Union sanctions.

In a previously unpublicised letter to MPs last year, Mr Cable acknowledged that his officials had authorised the export of an unspecified quantity of the chemicals in the knowledge that they were listed on an international schedule of chemical weapon precursors.

Downing Street insisted today that Britain’s system for approving arms exports to Syria is working even though licences for two chemicals capable of being used in making nerve gas were approved by the Government and blocked only by EU sanctions.

The Prime Minister’s official spokesman said: “You see the system working, with materials not exported. The facts are that the licences were revoked and the exports did not take place. The Prime Minister’s view is that that demonstrates that the system is working. There is a sanctions regime, which is a very active part.”

Critics of the Business Secretary, whose department said it had accepted assurances from the exporting company that the chemicals would be used in the manufacture of metal window frames and shower enclosures, said it appeared the substances had only stayed out of Syria by chance.

The shadow Business Secretary Chuka Umunna told The Independent: “It will be a relief that the chemicals concerned were never actually delivered. But, in light of the fact the Assad regime had already been violently oppressing internal dissent for many months by the beginning of 2012 and the intelligence now indicates use of chemical weapons on multiple occasions, a full explanation is needed as to why the export of these chemicals was approved in the first place.”

The Labour MP Thomas Docherty, a member of the Commons Arms Export Controls Committee, will today table parliamentary questions demanding to know why the licences were granted and to whom.

He said: “This would seem to be a case of breath-taking laxity – the Government has had a very lucky escape indeed that these chemicals were not sent to Syria.

“What was Mr Cable’s department doing authorising the sale of chemicals which by their own admission had a dual use as precursors for chemical weapons at a time when the Syria’s war was long under way?”

The licences for the two chemicals were granted on 17 and 18 January last year for “use in industrial processes” after being assessed by Department for Business officials to judge if “there was a clear risk that they might be used for internal repression or be diverted for such an end”, according to the letter sent by Mr Cable to the arms controls committee.

Mr Cable said: “The licences were granted because at the time there were no grounds for refusal.”

Although the export deal, first reported by The Sunday Mail in Scotland, was outlawed by the EU on 17 June last year in a package of sanctions against the regime of Bashar al-Assad, the licences were not revoked until 30 July. Chemical weapons experts said that although the two substances have a variety of uses such as the fluoridation of drinking water, sodium and potassium fluoride are also key to producing the chemical effect which makes a nerve agent such as sarin so toxic.

Western intelligence has long suspected the Syrian regime of using front companies to divert dual-use materials imported for industrial purposes into its weapons programmes. It is believed that chemical weapons including sarin have been used in the Syrian conflict on 14 occasions since 2012.

Mr Cable’s department last night insisted it was satisfied that the export licence was correctly granted. A spokesman said: “The UK Government operates one of the most rigorous arms export control regimes in the world.

“The exporter and recipient company demonstrated that the chemicals were for a legitimate civilian end-use – which was for metal finishing of aluminium profiles used in making aluminium showers and aluminium window frames.”